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Vacuum Pump Limit Switch System Answered

Hey Everyone... I'm Alive! I've just been really busy with my senior design project :D

So today I acquired ToolUsingAnimal's vacuum pump on loan. I'm going to be doing some vacuum bagging.... To prevent his pump from blowing out, I need to make a limit switch so the pump shuts down when it reaches full vacuum and then turns back on if some of the vacuum bleeds of somewhere.

So the method I cooked up today was to use a syringe with a weight. The weight will pull the syringe open and close a micro switch that turns the compressor on. If there's vacuum, the plunger will be fully retracted and the switch will be open (off position). This method will cost me all of $5 or so (10 cents for the syringe and $4 for the switch).

I was just doing some math on how much weight I'll need for different syringe sizes - and it's totally plausible (we're talking less than a pound to 3 pounds depending on syringe diameter).


Question - any other suggestions or blatantly obvious/easy/cheap solutions? I know this sounds Rube Golbergian, but it's not nearly as complicated as it sounds (or looks) :D Emphasis on cheap, I just put in an order for roughly $2000 of materials and supplies!

17 Replies

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trebuchet03 (author)2008-02-23

So here's what I've learned thus far... 1. Lubrication makes it more slippery - but increases adhesion forces significantly. So much so that the pump doesn't pull enough vacuum to overcome this. 2. The longer blade on an outlet is the neutral pole (a bit of info I've never been able to hold on to) 3. I found what may be a suitable extension spring So far, I've bought... 5' of 12 gauge 3 wire cable -- $4.80 1 16A, 125/220V micro switch with an awesome blue lit button! - $4.95 (don't need the button though - so I'll find another use elsewhere) 1 extension spring with hook ends - $1.50 2 machine screws to hold switch - $0.08 1 Female plug adapter - $3 2 flat push in blade connectors (for switch) - $0.40 I still need 1 male connector and some wood to hold everything together. What's nice is that this vacuum switch can be used for any vacuum pump. I was just told that one of my teammates has a fridge compressor that he used to vacuum out an a/c system. So we're going to measure which one pulls the most.

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gmoon (author)2008-02-23

Speaking of "spring-loaded..." Why not a spring (inside or outside of the syringe) instead of the weight? It's response to pressure would be less linear, but not sure that matters...

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Kiteman (author)gmoon2008-02-23

Using weights, he could adjust the level of vacuum being maintained.

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gmoon (author)Kiteman2008-02-23

Moving the switch would do the same...

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trebuchet03 (author)gmoon2008-02-23

I think I'm going to hard mount the switch and syringe. But I think I might go with a spring on the outside mounted to a screw, instead of weights. Then I can adjust vacuum by changing the tension on the spring. We'll see :)

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gmoon (author)trebuchet032008-02-23

Sure, vary the tension on an extension spring... I like the "weight approach," 'cause you could really fine-tune it (throw a few quarters on.) But once setup, moving it would be a big production, and small movements (adjusting hosing, etc.) could throw the whole thing a-kilter.

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Kiteman (author)2008-02-23

... Thinks ...

... Thinks some more ...

... Thinks really hard ...

This works for me. The only modification I could suggest would be to add a "reservoir" (such as an empty propane tank) in line 'twixt pump and whatever you're vacuuming, (can you have a reservoir of nothingness?) - then, if the system happens to provide very jerky changes in pressure, they will be smoothed out a little by the lag in the tank (I'm thinking kind of like a capacitor to smooth current).

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trebuchet03 (author)Kiteman2008-02-23

Yes, I'm working on locating a an old R-22 cylinder to use as a dump tank. Apparently they have easier to use fittings - benefits for not having mercaptain smelly stuff inside :D

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NachoMahma (author)2008-02-22

. Most vacuum pumps are designed to run "dead headed" for long periods of time, although I can't vouch for the pump you have. . You're only dealing with 15psi deltaP maximum and you don't need to be real accurate, so you don't need an expensive vacuum/pressure switch - something off an air conditioning system or a car engine might work. . A vacuum switch is just a pressure switch piped up "backwards."

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trebuchet03 (author)NachoMahma2008-02-22

Hrmm... Do you know what sort of vacuum a car a/c works with? If I can grab one of those sensors plus a relay - that'd make this easy :) But ya, it doesn't need to be accurate... It just needs to ensure vacuum is always there while preventing the motor from being continuously run. The engine vacuum thingers I've found are diaphragms that pull a metal rod. That might work too...

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NachoMahma (author)trebuchet032008-02-23

. Car manifold vacuum usually maxes out around 30 inches of water, IIRC. But I'm used to working on old carbureted engines. . Not sure about the a/c. Come to think of it, they have low pressure cut-out switches, not vacuum switches. . If you can find a 10-15psi pressure switch that has both sides of the diaphragm piped, you can just tube it up backwards. Or seal up the atmospheric side so you can tube up the vacuum to it.

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zachninme (author)2008-02-22

I've thought of a few different switches, but they're all nearly the same as your setup. On a different note, what about also pulling a vacuum on some other bag with a one-way valve, and a pressure relief valve. When it can't suck any more on the bag, it will go to the other bag... the valve would have to be just under air pressure, though.

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ry25920 (author)2008-02-22

Flip a burst disk around backwards. If you got to long, It will pop and allow air in. They work. They keep my C02 tank from blowing.

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trebuchet03 (author)ry259202008-02-22

A burst disk will only work once.... I need this to cycle on off on off etc. I'm not worried about a tank blowing (as in the case of a CO2 cylinder) - I just don't want the pump running when unnecessary.

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ry25920 (author)trebuchet032008-02-22

They're spring loaded. At least they are on my tank.

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trebuchet03 (author)ry259202008-02-22

Oh, you're talking about a pressure relieve valve - not a burst disk (I know, it makes a loud sound when triggered...). That would work if it didn't relieve vacuum (which I'm trying to hold).

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ry25920 (author)trebuchet032008-02-22

I guess so. My tank blew it's disc in the middle of the game. HOLY CRAP!!!!! The cloud of C02 that came off of my gun was 6 feet in diameter. The Ref who was trying to fix my gun got a little bit of frost bite

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